The present invention relates to a valve system or valve mechanism for an internal combustion engine, particularly comprising a valve seat and a valve member having an improved wear and abrasion resistance performance.
In a conventional art, many of internal combustion engines such as of an automobile utilize various kinds of valve seats made from an iron-base (or -based) sintered alloy, and many studies and searches have been performed for improving the wear and abrasion resistance of the valve seat.
In order to improve the wear and abrasion resistance of the valve seat, a prior art provides a method of dispersing hard particles such as Fe--Mo or Fe--W into a base member of the valve seat. In this method, however, in order to improve the wear and abrasion resistance of the valve seat by increasing containing amount (content) of the hard particles, there occurs a problem that a valve member as a counterpart to the valve seat is violently worn.
In order to obviate such problem, there has been studied a valve seat for improving the wear and abrasion resistance, as well as reducing an attacking property against the counterpart, for example, as disclosed in Japanese Patent Laid-open Publication No. HEI 5-43913. This prior art publication discloses a valve seat made from an iron-base sintered alloy prepared by dispersing, into a base member of the iron-base sintered alloy, spherical carbide dispersed type hard particles and/or intermetallic compound dispersed type hard particles which have Micro Vickers hardness of Hv500 to Hv1800 at a ratio of 5 to 25 mass % with r reference to the total mass of the base member. In a case where the valve seat of such structure is used for a valve system of a fluid fuel engine such as using gasoline or Diesel Fuel, lubrication property (lubricity) between the valve member and the valve seat can be maintained by a fuel or combustion product (such as carbon (C)), so that the abrasion between the valve seat and the valve member is suppressed.
Incidentally, in the conventional valve system of a fluid fuel engine, there is mainly used, as an intake side valve system, a valve system comprises a valve seat made from an Fe--C group sintered material and a valve member made from a quenched (hardened) martensitic steel corresponding to SUH1 (JIS G 4311, 9% Cr--3% Si--0.4% C--Fe (balance or remainder), and there is also mainly used, as an exhaust side valve system, a valve system comprises a valve seat infiltrated with copper or copper alloy to a sintered material having a base member of a high-speed tool steel (corresponding to SKH 51) and a valve member formed by forming a padding layer of Stellite No. F (Trade Name: DELORO STELLITE Co. Ltd.) to a valve face of austenitic steel corresponding to SUH35 (JIS G 4311, 21% Cr--4% Ni--9% Mn--0.4% N--0.5% C--Fe (balance)).
However, in an engine using a gas fuel such as natural gas, in comparison with an engine using a fluid fuel, an abrasion between the valve seat and the valve member easily progresses due to intermetallic contact therebetween, and flow due to plastic deformation, adhesion wearing or sliding wearing is caused because of reasons of: (1) no cooling function and lubricating function due to the nature of the gas fuel itself; (2) high environmental temperature in the engine; (3) inferior lubricating function due to less combustion product; (4) inferior lubricating function due to less iron oxide product; and (4) easy generation of corrosion, particularly in the case of CNG (compressed natural gas).
In spite of the above fact, in the prior art, a study or countermeasure in a case of many intermetallic contacts such as caused in the gas fuel engine has not been fully considered and the valve system for the fluid fuel engine was applied to the valve system for the gas fuel engine.
In the case where the valve system for the fluid fuel engine is applied as it is to that for the gas fuel engine, in the intake side valve mechanism, the valve seat, made from the Fe--C group sintered material, is inferior in the lubricating function at an environmental temperature of about 150 to 250.degree. C., and the valve member quenched with the martensitic steel corresponding to SUH1 is inferior in the lubricating property and hardness, thus providing a problem. Further, in the exhaust side valve system, the valve seat made by infiltrating Cu to the sintered material having the base of high-speed tool steel in inferior in self-lubrication property and causes an adhesion abrasion due to the intermetallic contact, and the valve member formed by forming a padding layer of the Stellite No. F to the valve face of the austenitic steel corresponding to SUH35 is inferior in strength against high temperature, thus also providing a problem.